Ocean Salmon Distributions

This project extends and advances existing ocean distribution and size models for Chinook Salmon, a major prey of Southern Resident Killer Whales (SRKW) and target of important fisheries, to provide ocean distribution estimates for multiple run-types (fall, summer, and spring Chinook) arising from rivers from California to Alaska by season and under variable oceanic conditions. It leverages very large tag-recapture databases that have been developed for Chinook Salmon over the past 50 years – hundreds of millions of fish tagged and millions recovered – and links these recoveries to a range of fisheries in which Chinook are targeted or captured as bycatch. It integrates data coast-wide, from Alaska to California, and over more than 30 years (1978-2015), to provide a first synthetic, quantitative description of Chinook distribution that can be used to understand the total Chinook prey field available to SRKW, fishers, and other predators in different seasons and under alternate ocean states. In addition, this projects examines long-term trends in Chinook salmon size and their biological implications. Chinook populations have shown pronounced trends toward smaller and younger fish returning to spawn, and these trends have accelerated in the last 15 years. This erosion of the age-size structure and life-history

diversity may negatively affect population productivity via reductions in reproductive potential, and may compromise the long-term viability of populations and jeopardize the sustainability of Chinook salmon fisheries. Consequently, long-term shifts in life-history characteristics, which are likely caused by changing ecological conditions in the ocean, might need to be accounted for when estimating reference points for fishery management. This work supports ongoing efforts to recover SRKW populations, informs the SRKW critical habitat designation process and recovery plans, feeds into the PFMC SRKW ad hoc work group, and is directly in line with the NMFS Ecosystem-Based Fisheries Management Road Map and Policy as well as the National Climate Science Strategy.

Chinook Salmon (Oncorhynchus tshawytscha) are an important target of recreational and commercial fisheries species and the predominant prey item of endangered Southern Resident killer whales (SRKW). Chinook salmon have been closely tied to the population viability and demographic rates of both SRKW and other populations of killer whales that inhabit the Northeast Pacific Ocean. Identifying strategies for increasing available Chinook is complex – many populations of Chinook Salmon in the Pacific Northwest are themselves threatened or endangered and are prey for other predators. Furthermore, the availability of Chinook for fisheries and SRKW depends not on overall Chinook abundance but on the spatial and temporal overlap between Chinook, fishing fleets, and SRKW. Despite decades of Chinook fisheries and research, the ocean distributions of many Chinook populations remain poorly documented and a key uncertainty for management

Existing models of salmon ocean distribution are relatively coarse spatially and do not account for differences among seasons. While environmental conditions in the ocean have been linked to Chinook population dynamics, there are no coastwide estimates of how Chinook distributions shift in response to oceanographic variables such as ocean temperature, making longer-term projections of Chinook distribution in a changing climate difficult. As SRKW and multiple fisheries utilize different habitats and locations in different seasons and must respond to longer term shifts in prey availability, understanding both short- and long- term availability of Chinook for SRKW depends upon understanding ocean distributions of Chinook by season and life-history type.

This project will use advanced spatio-temporal modeling to provide ocean distribution estimates of the dominant life-history types of Chinook across the Northeast Pacific from California to Alaska. It will build on an existing modeling framework that has been applied to fall Chinook (Shelton et al. 2019), but will extend the model to include (1) spring and summer Chinook, (2) extend the ocean distribution to central and western Gulf of Alaska, and (3) include oceanographic variables that may predict distribution patterns across space and time. The results can be used to describe Chinook prey fields available to fisheries and SRKW throughout the northeast Pacific. By including variables such as sea surface temperature, the model will be able to incorporate global forecasts (e.g. CMIP5; http://cmip-pcmdi.llnl.gov/cmip5/) to predict future changes of ocean distribution in response to expected climatic variability. Additionally, by incorporating Chinook data from several North Pacific fisheries, it will extend the spatial range of the model, thereby developing a more comprehensive understanding of the distribution of the sources of Chinook fishing mortality, which is of central importance to the Pacific and North Pacific Fishery Management Councils, and Chinook management within the Pacific Salmon Treaty Area.

With regard to Chinook salmon size, our goals are to (i) assess the effects of changing ocean conditions on the mean size and age of Chinook salmon returning, (ii) estimate historical trends in mean fecundity and population egg

potential, (iii) evaluate consequences of changing reproductive potential for estimating management parameters, and (iv) conduct a management strategy evaluation to guide future management. In short, we seek to understand how information on life-history changes can improve the management of this iconic species and sustain the fisheries it supports.

Declines in reproductive potential are expected to reduce population production unless spawner abundances increase sufficiently to offset the reduction in total egg potential. Reductions in population production may negatively affect the long-term viability of Chinook salmon fisheries (Healey and Heard 1984; Calduch-Verdiell et al. 2014) because the harvest that can be sustained by a population may decrease continuously over time, or because management models fail to

account for reductions in reproductive potential and thus underestimate sustainable escapement targets.

Moreover, changes in Chinook salmon production can affect other fisheries, for instance by changing bycatch

limits in the Bering Sea pollock fishery. Our work will clarify to which degree escapement targets for

Chinook salmon should be changed given past and future declines in escapement quality. In situations with considerable uncertainties in data and understanding, it is unclear whether changes in management reference

points are needed to improve the likelihood of management successes. Our management strategy evaluation will clarify these issues and will inform management agencies about needs and best strategies for effectively

dealing with changing demography of Chinook salmon.

Revise and publish historical ocean distributions and projected future distributions for major fall Chinook populations in California and the Columbia basins.

Generate publication that provides estimates of ocean distribution for spring and summer run Chinook from California to Alaska.

Collect necessary data from Alaska to California and implement a new model that can incorporate data from both coded-wire tags and genetic stock identification to improve estimates of ocean distribution and survival of Chinook salmon.

Implement a simulation model for Chinook to understand how changes in ocean temperature are expected to shift the aggregate abundance and stability of Chinook among regions.

Begin development of public facing products that can be accessed by managers and the public to visualize ocean distributions in a changing ocean.